In a conventional data delivery system, a delivery device divides content data to be delivered into blocks each of which is a predetermined size. The delivery device sequentially transfers each unit data of one block of the content data to a communication terminal. The communication terminal sequentially receives the unit data, which is one block, from the delivery device. The communication terminal then memorizes a block ID which has been associated with the received unit data.
By the way, in a case in which the communication between the communication terminal and the delivery device is broken before the whole content data is delivered and the delivery was stopped, the communication terminal notifies, after the communication is recovered, the delivery device of the block IDs of the unit data that the communication terminal has already received.
And then, the delivery device recognizes, based on the block IDs, the already-delivered blocks and still-not-delivered blocks of the content data, and then transmits the unit data including the still-not-delivered blocks to the communication terminal. This prevents the delivery device from transmitting the same blocks to the communication terminal. And that allows the delivery device to transmit the still-not-delivered part of the content data to the communication terminal (see Patent Document 1, for example).    Patent Document 1: Japanese Patent Publication No. 2002-135350 (Pages 1, 6, and 11, and FIGS. 1 and 17)